1. Field of the Invention
The present invention relates to a tuning apparatus for use in a television receiver, video tape recorder, and the like.
2. Description of the Prior Art
FIG. 25 is a block diagram showing an example of the structure of a prior art electronic tuning apparatus (synthesizer tuner). The operation of this apparatus will be described below.
To the antenna input terminal 1 are input television signals of the VHF band and UHF band (television broadcast signal or reproduced radio-frequency (RF) signal from a video tape recorder). The television signal of the VHF band is input to a bandpass filter 2 of a VHF circuit VC and, when it is set so that a television signal of the VHF band is received, the television signal of the desired reception channel is selected in a tuning circuit 3. The thus selected desired television signal is amplified in an RF amplifier 4, enhanced for selectivity in a tuning circuit 5, and input to a VHF mixer 6. This received signal is mixed with an oscillation signal from a VHF local oscillator 7 in the VHF mixer 6 to be converted into an intermediate frequency (IF) signal. The IF signal is amplifier in an IF amplifier 8, passed through an IF filter 9, and output from an IF output terminal 10. In like manner, when it is set so that a television signal of the UHF band is received, the desired reception television signal is selected from the television signals of the UHF band in a tuning circuit 12 of a UHF circuit UC. The thus selected desired television signal is amplified in an RF amplifier 13, enhanced for selectivity in a tuning circuit 14, and input to a UHF mixer 15. This received signal is mixed with an oscillation signal from a UHF local oscillator 16 in a UHF mixer 15 to be converted into an IF signal. This IF signal is input to the IF amplifier 8 through a bandpass filter 17, amplified in the IF amplifier 8, and passed through the IF filter 9 to be output from the output terminal 10. Each of the outputs of the VHF local oscillator 7 and UHF local oscillator 16 is input to a PLL circuit Pl constituted of a prescaler 18, programmable divider 19, and phase comparator 20. Reference numeral 21 denotes a channel selector for selecting a desired channel.
FIG. 26 is a diagram showing a structure of a portion of a prior art tuning apparatus of the described type. Referring to the diagram, reference numeral 101 denotes a transistor for amplifying a signal of the VHF band, 102 denotes a transistor for amplifying a signal of the UHF band, 103 denotes a transistor which is turned on when the VHF band is selected, 104 denotes a transistor which is turned on when the UHF band is selected, 105 and 106 denote resistors, and 107 denotes a terminal to which a D.C. power is supplied.
As described above, prior art tuning apparatuses were provided with two mixers, i.e., one mixer 6 for the VHF band and the other mixer 15 for the UHF band. However, the provision of two mixers not only made the circuit configuration complex but also produced many wire crossovers when the apparatus was arranged in an integrated circuit (IC) array, whereby stray capacitance was introduced into the circuit and the impedance matching was made difficult, and in consequence, obtaining good performances from the apparatus was rather difficult.
Further, since the VHF local oscillator 7 and the UHF local oscillator 16 were directly connected to the prescaler 18 to supply the same with the local-oscillator signal, closed loops were formed by the prescaler 18, two local oscillator 7, 16, and two mixer 6, 15. Hence, there was a problem of higher harmonics from the prescaler 18 penetrating into the local oscillators and mixers thereby causing unstable operation and beat interference, and the configuration was particularly unsuitable when it is realized on an IC.
Furthermore, the total gain of the tuning apparatus when it receives the VHF band is greater than that when it receives the UHF band. Thus, when attempting to compensate for the difference in the gains wholly by an automatic gain control (AGC) operation in the RF amplifier 4 of the VHF circuit VC, a problem of deterioration in the noise factor (NF) occurred.
In addition, the band switching method for switching between the UHF and VHF bands in the prior art tuning apparatus by tuning on/off switching transistors 103, 104 between the amplifying transistors 101, 102 and the power supply terminal 107 such a band switching method had the following problems:
1 When the transistor 103 is turned on, the transistor 103 and resistor 105 are connected in parallel with the load of the transistor 101. The impedance of the transistor 103 varies as the collector current of the transistor 101 is changed. Consequently, in this circuit, the load impedance of the transistor 101 changes depending on the collector current.
2 The transistor 104 is turned off when the transistor 103 is turned on, but since the transistor 104 has stray capacitance in this case, the circuit including the transistor 102 is not completely isolated from the circuit on the VHF side in terms of radio frequency, and thereby, a bad effect is produced on the received signal of the VHF band.
The same is also true when the transistor 103 is turned off and the transistor 104 is turned on.
3 While NPN type transistors having good RF characteristics are used for amplifying transistors 101, 102, it is necessary to use PNP type transistors for the switching transistors 103, 104 on account of the polarity of the power supply thereto. When such circuits including two kinds of transistors are arranged in an IC array, the fabrication process becomes complex.